The present invention relates to reflector antennae apparatus and more particularly to apparatus for minimizing the effect of aperture blockage.
In antennae systems where the radiant energy is transmitted by one or more feedhorns toward a reflecting surface, the energy reflected by such surface is partially blocked by the feedhorn structure; and may be blocked, in many instances, by other structures that are in the path of the reflected wave energy. Such blocking is commonly referred to as aperture blocking; and is an important factor in the increase of sidelobe level, as well as the reduction of gain, and the sharpening of the beam. Although aperture blocking occurs in various well-known geometrically shaped antennae reflectors, such blocking is more serious in the well known parabolic cylinder type reflector. Such parabolic cylinder reflector type antennae offers certain advantages over other type reflectors in that they can be fed not only by a point source but by a line source feed that may take many forms, such as linear arrays of dipoles of waveguide slots, for example.
In reflector type antennae, it is the customary practice to decrease to the extent feasible the surface area of the feed structure facing the reflecting surface; and of course, to minimize the presence and size of any other structure in the path of the reflected waves. However, for antennae systems that include a number of feedhorns, or in line feed sources, the surface area facing the reflector may be relatively small but have a considerable dimension that is parallel to the reflected waves, which is referred to herein as the depth of the blocking structure. Further, since multiple feedhorns are utilized, the angles of incidence of the reflected waves differs in accordance with the particular feedhorn that is radiating the energy.
Heretofore, one of the joint inventors herein, namely Coleman J. Miller, proposed an assembly for reducing the effect of feed blockage in a parabolic cylinder reflector antennae wherein an etched circuit board or structure having a plurality of spaced exposed conductive plates in the form of elongated rectangles was secured to the feedhorn structure in a position such that the exposed surface of the circuit board lay in a plane parallel to the direction of the reflected waves. The conductive rectangles were arranged in rows with the bottom edge of the rectangles of one row being coincident with the top edge of an adjacent row. The rows were staggered so that each conductive rectangle of one row was positioned intermediate adjacent rectangles of the next row. Such conductive rectangles had a dimension of approximately 0.0826 wavelengths in the direction of the reflected waves, and a dimension of 0.165 wavelengths in the direction of the E-field. The conductive rectangles of each staggered row were spaced apart center to center in the dielectric surface or in other words in the direction of wave propagation 0.3304 wavelengths. The perimeters of each such conductive rectangles measured approximately 0.5 wavelengths.
Although structures such as previously described provided improvement in the sidelobes, their effectiveness decreased for different angles of incidence of reflection. Also, it was necessary to manufacture and assemble such boards for positioning in the proper direction relative to the reflected wave; that is, with a narrow portion of the exposed rectangular conductive portions extending in the plane or direction of reflection of the wave, with the longer dimension of the rectangle parallel to the E-plane of the reflected wave.
Thus, it is desirable to provide an improved assembly for limiting the effect of aperture blockage to reduce the sidelobes; and yet permit versatility in assembly and manufacture while effectively reducing such blockage for all angles of incidence of the reflected waves.